The fabrication of integrated circuits (IC) in the semiconductor industry typically employs plasma discharge, especially CCP discharge, to create and assist surface chemistry within a processing chamber necessary to remove material from and deposit material onto a substrate.
Wafer sizes continually increase, and after transition from the diameter of 300 mm, the wafer sizes in microelectronics fabrication today have a diameter of 450 mm. Thus, the surface area of a wafer has increased 2.25 times, and therefore it becomes more difficult to provide plasma density needed for etching processes.
For a 450-mm wafer, the scaling method that was used successfully during the previous periodical increase in wafer size does not work anymore because the proportional increase in power from an RF generator will lead to extra high voltage and electrical breakdown and arcing in the plasma processing chamber.
One solution to this problem is to increase the frequency of a capacitive discharge. This improves power coupling to the plasma because of the decrease in sheath screening. High-frequency excitation allows a much higher rate of ion flux (i.e., plasma density) when compared with classical 13.56-MHz excitation at the same RF power. However, at frequencies much higher than 13.56 MHz, the electromagnetic effects cause some problems that lead to the deterioration of uniformity in plasma density. Such nonuniformity may be caused mainly by the following three factors: (1) a standing wave effect that enhances RF power deposition at the discharge center; (2) an edge effect known as a “telegraph effect” that creates reflection of RF power from the edges and causes some plasma density perturbation from the edges; and (3) a skin effect that enhances RF power deposition near the edges of the showerhead and tends to increase local plasma density in the vicinity of the edges.
These factors cause drastic changes in distribution of plasma density and ion flux and eventually result in nonuniform etching. Therefore, enhancement of uniformity is a key consideration for transition to high-frequency excitation.
CCP reactive ion etching tools usually use a dual frequency model that originates from the desire to separately control the magnitude of ions and radical flux, on one hand, and ion energy distribution to the wafer, on the other hand. A reactive ion etching system with dual frequency typically consists of a parallel-plate plasma-etching chamber wherein the CCP discharge is generated between an upper electrode, or cathode, and a lower electrode with a wafer. A conductive silicon wafer is held by an electrostatic chuck and is surrounded by a silicon focusing ring and a dielectric outer ring. The wafer, the electrostatic chuck, and the focusing ring are combined into a wafer system. Both electrodes are joined through matching networks to separate RF generators with a frequency ratio from (10:1) to (10:5), wherein a higher-frequency generator is connected to the upper electrode (cooler plate), and another generator that operates at a lower frequency is connected to the wafer system through a blocking capacitor.
The showerhead is made from a conductive high-purity material such as a single crystal silicon, polycrystalline silicon, or silicon carbide. A chemically reactive gas such as CF4, CHF3, CClF3, SF6, or a mixture thereof with O2, N2, He, or Ar is introduced in the plasma processing space and maintained at a pressure that is typically in the millitorr range.
Due to the difficulty in drilling gas holes having a high length-to-diameter ratio through a relatively thick showerhead plate of silicon, the gas holes are formed by first forming the countersinks on the back side of the showerhead and then drilling relatively narrow passages of approximately 0.5 mm in diameter through the remaining portion of the showerhead plate. This diameter is found to provide an indispensable gas flow rate. On the other hand, due to such a small diameter in spite of a matrix of holes, the surface of the process side of the showerhead remains relatively smooth, and the sheath plasma uniformity is not deteriorated either by the matrix structure of such a surface or by surface roughness. As a result, under high pressure developed in the gas pressure reservoir between the showerhead and the cooler plate, the gas flow that is fed through the gas-feed passages of the cooler plate redistributes some pressure to the lower-pressure side so that this redistributed pressure penetrates into the plasma processing chamber, resulting in uniform flow-rate distribution across the entire plasma processing chamber.
One characteristic that is generally required for the gas delivery system is strict control of passage dimensions and spacing between the gas holes of the showerhead so that uniform gas distribution is maintained on a particular surface area of the showerhead. The showerhead is a consumable part that is supposed to be replaced periodically because etching and ion bombardment during plasma processing erodes the side of the showerhead that faces the plasma. Usually, instead of replacing the showerhead, the eroded part of the process side is removed by polishing, after which the process side of the showerhead can serve for two more additional terms. An electrical and mechanical contact between the cooler plate and the showerhead is provided through the periphery portion of the showerhead backside. In other words, RF voltage is applied to the periphery of the showerhead backside, and because of the recess in the cooler plate, the central part of the showerhead does not contact the cooler plate. Thus, RF power propagates from the edges in the radial inward direction to the center of the showerhead through the thin layer on the process side of the showerhead, which is made from silicon. Generally, surface resistance of silicon is approximately 200 Ohm/cm2. Therefore, at the conventional frequency of 13.56 MHz, surface impedance allows launching of the electromagnetic waves into the plasma from the total surface of the showerhead rather than from edges where the showerhead is connected to the cooler plate.
For small-diameter showerheads, e.g., those intended for processing 300-mm wafers and operating at low frequencies of RF power, e.g. 13.56 MHz, the transfer of power from the cooler plate to the showerhead through the edges, i.e., through the areas of contact between the cooler plate and the showerhead, does not drastically affect the uniformity of the plasma generated under the process side of the showerhead in the processing space of the plasma process chamber. This is because the wavelength of RF power is much larger than the diameter of the showerhead, and in this case RF power uniformly propagates from the edges to the center and is transferred to a plasma discharge from the total surface of the showerhead into the conductive bulk plasma. This discharge ionizes and dissociates the reactive gas that forms plasma, thus generating ions and chemically active radical particles. The ions strike the surface of the wafer that is to be etched by chemical interaction and momentum transfer. Because ion flow is predominantly normal to the surface of the wafer, the process produces well-defined vertically etched sidewalls. The highly reactive radicals are not charged and can penetrate even into the narrow and deep trenches on the wafer and provide etching there. Ion bombardment energy is influenced by excitation in the plasma sheath adjacent to the wafer because low-frequency voltage is applied to the bottom of the chuck (lower electrode).
Thus, the level of power introduced into the system at low frequency provides control of coordinates and angular distribution of ion energy across the surface of the wafer. However, when high-frequency power is applied to the showerhead (upper electrode) from the cooler plate through the zone of contact with the showerhead, plasma density is controlled by high currents that are displaced more significantly toward the aforementioned zone of contact and increase the Ohmic power transferred to the plasma and cause heating of the plasma sheath. In other words, under the conditions described above, RF power of high frequency is responsible for generating ions and radicals. Because the system operates at dual frequencies, plasma density and the ion bombardment energy can be adjusted separately.
RF power is supplied from an RF power supply unit through a matching network and the cooler plate to the backside of the showerhead specifically through the periphery of the latter (as mentioned above, the central part of the cooler plate is occupied by a recess). Power is transmitted through the process-side showerhead surface to the plasma. Higher frequency causes greater intensity of the electrical field at the central part of the showerhead working surface than at the peripheral portion of the showerhead. Therefore, the density of generated plasma is higher at the central part of the process space than at the peripheral portion of the process space. As a result, the uniformity of the plasma density further deteriorates, which results in poor planar uniformity and charge-up damage to the plasma etching.
With an increase in showerhead size and much higher frequency required to support the optimal level of plasma density to maintain uniformity at dual frequency, the CCP plasma etching systems become more complicated because of the electromagnetic and finite wavelength effects that deteriorate this uniformity. The main source of plasma nonuniformity at ultra-high frequency is the so-called standing wave effect.
At extra-high frequencies, RF voltage applied to the rear peripheral side of the showerhead is concentrated mostly at the edges and does not propagate to the center through the surface layer of the showerhead. At such frequencies, impedance of a plasma sheath is lower than the impedance on the surface of the showerhead. Therefore, RF power directly enters the plasma, specifically, the plasma sheath in the vicinity of the edges. After entering the plasma, the electrical field does not significantly penetrate the plasma but appears to be wave-guided in the sheath because Ohmic resistance—which in this case exists in the central part of the showerhead—is resistant to high frequency.
As RF frequency increases, the plasma-effective wavelength decreases, and therefore uniformity in the electrical field worsens. At 150 MHz, the showerhead, and, hence, the electrode radius, is larger than the quarter wavelength. In this case, RF power applied to the periphery leaves the process-side surface of the showerhead and propagates directly into the plasma sheath. Therefore, the RF voltage is wave-guided in the sheath that is adjoined to the surface of the showerhead. Because the plasma-effective wavelength decreases, the size of the showerhead becomes comparable with or less than the size of the wavelength. As mentioned above, at 150 MHz the showerhead radius is larger than the quarter wavelength, and in this case the phase change of the RF power from the edge to the center of the showerhead also becomes greater than the quarter wavelength. However, use of sufficiently high frequencies (short wavelengths) is accompanied by occurrence of some constructive and destructive interferences and skin effects. Because of constructive interference of counter-propagating waves from the opposite sides of the showerhead, the amplitude of the electrical field in the sheath increases at the showerhead center. This causes nonuniform distribution of the plasma density, with higher plasma density in the center than at the edges. Therefore, depending on the level of frequency, the finite wavelength produces nonuniformities, which are already problematic even for 300-mm showerheads, and become highly problematic for 450 mm showerheads. Thus, at the frequency of 150 MHz, a transition occurs from a traveling wave to a standing wave, whereby at each point along the showerhead radius, the RF power oscillates at a close phase. This phenomenon causes interference of the aforementioned wave with the counter-propagating waves reflected from the rear side of the showerhead.
The electrical field launched by the RF power and the plasma current introduced into the plasma becomes highly nonuniform, and the amplitude of the electrical field in the plasma sheath increases at the center of the showerhead (electrode). Several simulations made by different authors show that the electrical field is maximal at the center of the discharge and decays toward the edges, thereby following the Bessel function. Such changes in RF power distribution result in nonuniform RF power deposition into the plasma. As a result, the wafer treatment processes such as etching or deposition become nonuniform as well.
Local deposition of RF power in the plasma that occurs near the edges of the showerhead and the chuck (electrodes) leads to increase in local plasma density at the edges and is referred to as the skin effect.
For a CCP reactor with the geometry described above, argon plasma at an RF power frequency of 150 MHz (450 W) is sustained at a gas pressure of 50 mTorr. Under these conditions, ion flux density along the showerhead radius has the following values: the plateau around the center and up to a radius of 50 mm in the showerhead has ion flux density equal to I=4.75×1015 cm−2 s−1; the lower plateau in the area from radius 150 mm up to the edge that has a total radius of 240 mm has ion flux density equal to I=1.75×1015 cm−2 s−1; and the linear downfall branch has ion flux density I decreasing with radius R and expressed by the following formula:I=4.75×1015 cm−2 s−1(1−bR),where b in the area with the radius from 50 mm up to 150 mm is ˜2.75×10−2.
In the above structure, the finite wavelength effect weakens by decreasing the conductivity of the plasma-contacting surface of the electrode from the edge to the center of the electrode by applying a coating of dielectric material with variable density. In this case the RF wave can readily propagate through the surface of the showerhead dielectrics rather than through the plasma, especially at the center of the showerhead with the lowest conductivity. Thus, the peak of electron density at the center is diffused, and uniformity becomes smoother.
A drawback of this method is erosion and sputtering of the dielectric layer during the plasma process. As a result, plasma density distribution will change from process to process. Another drawback is contamination of the wafer by the sputtered material.
Other methods to suppress the effect of electromagnetic waves were proposed. For example, Japanese Unexamined Patent Application Publication (KOKAI) No. 2000-323456 published on Nov. 24, 2000, inventor A. Koshiishi, describes a plasma processing device wherein the showerhead consists of two parts, and the central part of the showerhead is made from a material of high resistivity for consuming more RF power due to Joule heat. As a consequence, electrical field intensity is reduced to a greater extent in the central part than at the peripheral portion of the showerhead. This effect is used to level the distribution of plasma density. However, the high resistivity part of the showerhead consumes too much RF power as Joule heat, and this reduces the efficiency of the device.
Another method to improve uniformity of the ion flux incident onto the wafer is to use the so-called slot antenna. U.S. Pat. No. 8,080,107 issued to W. Kennedy, D. Jacob on Dec. 20, 2011 describes a showerhead that consists of two to six separate segments arranged in a ring configuration, such as segments of single crystal silicon. However, Yang Yang and Mark J. Kushner (see Journal of Applied Physics 108, 113306, 2010) suggested splitting the RF power and power at these segments at different phases. At the segments, the phases of RF voltage alternate with 180°. The in-phase excitation retains the character of a surface wave propagating along the sheath and thus higher-density plasma is formed in the center. However the out-of-phase excitation shifts the maximum plasma density from the center to mid-radius. This middle-peaked plasma density may lead to excitation of a higher order of waveguide mode in the chamber. As a result, adjusting the uniformity of the plasma density becomes more difficult.
A drawback of this method is complicated real-time control of plasma uniformity, which includes tuning of phases by oscillating the phases of the segments or the phase swapping to shift the pick of RF power distribution from the center to the middle. The metal ceramic at the process-side surface of the showerhead deteriorates the plasma sheath, and the resulting sputtering and erosion contaminate the product.
Sansonnens and Schmitt (see L. Sansonnens and J. Schmitt, Appl. Phys. Lett. 82, 182, 2003) proposed to solve the problem of plasma-density nonuniformity by fabricating a Gaussian-shaped surface profile on electrodes covered with a thin dielectric plate to confine the plasma in a constant interelectrode gap. In this proposal, the dielectric lens should have a Gaussian shape in order to receive a uniform voltage across discharge and thus suppress the standing wave effect. However, manufacture of a showerhead with an accurate and smooth curvilinear surface is an extremely complicated, inefficient, and expensive procedure.
There exists many other methods and devices for improving uniformity of plasma density distribution in a plasma processing cavity of a CCP processing apparatus. However, in the majority of cases these methods and apparatuses are aimed at solving the above problem by managing the distribution of RF power.
In this regard, the transition to 450-mm wafer etching systems according to the above methods and constructions are less efficient than methods based on controlling gas distribution. There are large numbers of gas holes of the same geometry in the showerhead for introduction of a process gas from the gas reservoir to the plasma processing chamber. The diameter of the gas holes is approximately 0.5 mm. Separations between the neighboring gas holes may vary from 5 mm to a greater distance. The rate of gas flow through each hole is the same. However, changing the geometry at the exits of the holes on the process side of the showerhead is not recommended (refer to U.S. Pat. No. 6,333,601 issued to S. Wickramanayaka on Dec. 25, 2001). It is taught that with an increase in gas hole diameter to a value greater than 0.5 mm, the process plasma will penetrate deeply into the hole and will increase the erosion rate at the hole exit.
It is known that positive ions of plasma accelerate toward the showerhead surface and bombard the surface. These ions gain high energy, especially in the vicinity of sharp edges where the density of the electrical field is high, so that the bombardment of ions on the surface causes sputtering. According to this theory, the sputtering damage is higher at the exits from the gas holes since plasma density at these places is higher. This process causes an extruded erosion of the gas hole compared to the other areas of the showerhead, resulting in enlargement in the diameter of the gas holes. With the increase in diameter, the plasma tends to confine in the vicinity of exits from these holes due to multiple reflection of electrons from the walls of the gas holes. Accordingly, with the increase in plasma density, the erosion rate in the gas holes accelerates. This process leads to tapering of gas holes, and eventually the total process-side surface of the showerhead should be re-polished.
In order to avoid degradation at the gas hole exits, all gas distribution enhancing means should be provided at the back side of the showerhead. A conventional method (Lam® Research) is to divide the gas pressure reservoir into several separated zones. For example, as disclosed in US Patent Application Publication 20100252197 (inventors Babak Kadkhodayan and Anthony De La Llera; published Oct. 7, 2010), the gas pressure reservoir is divided into two zones, where about 60% of the gas holes are in the inner zone and preferably about 40% of the gas holes are in the outer zone. These zones are separated from each other by a gas sealing element such as an O-ring. Thus, in order to optimize etching uniformity, the inner and outer zones must undergo plasma etching at different flow rates of the process gas, but during wafer processing, such as plasma etching, the showerhead and cooler plate heat up, and differences in coefficients of their thermal expansion place high loads on the O-ring. The O-ring is also exposed to a highly corrosive etching gas. As a result, the O-ring deteriorates and contaminates the etching process. Because of gas leakage through the deteriorated O-ring, the pressure in each zone is out of control, and the etching process further deteriorates.
Instabilities in the RF circuit that may be caused by many reasons may lead to occurrence of high frequency in RF power. RF power with extra-high frequency tends to propagate from the cooler plate to the showerhead through gaseous gaps and the gas reservoir rather than through the Ohmic contact surface on the periphery. A valuable part of energy is diverted by this capacitor type of resistance from the Ohmic resistance at the periphery. The gas in the reservoir is also susceptible to breakdown. A corona discharge and arcing also occur in this area. Further, the above-described abnormal conditions lead to a phenomenon that is known as a hollow cathode discharge, which occurs on the developed surfaces at the entrances to the aforementioned gas passages and penetrates inside the gas holes. This leads to the loss of power and distortion in passage geometry, and hence, to instability in the technological parameters of the process. Under the effect of the hollow cathode discharge in the worst case, the gas flow becomes totally ionized or becomes the carrier of charged particles that are introduced into the process plasma and can be converted into miniature arcs. The arcing overheats the inner part of the gas passages of the exit areas and changes the structure of silicon. This leads to drastic lowering of the resistance of silicon to sputtering and etching. The heavy ions that are generated in the process plasma bombard and sputter the overheated edges of the gas channels. They even develop craters that may reach 3 mm in depth or more.
Radicals penetrate the passage deeper than the ions and expand the initial diameter of the channel by two to three more times. In other words, it can be assumed that the aforementioned degradation of surface of the showerhead that faces a plasma discharge in the etching process can be explained by interaction of the charged particles and radicals on the plasma-surface boundary of the showerhead.
A source of deterioration of the showerhead surface is ionization of gas flowing through each gas, which is capable under some conditions to generate its own plasma discharge. Such a discharge, in turn, generates ions that bombard the passage wall. Moreover, in case of mismatching of impedances in RF power supply with the process system, the ionized gas flow can be easily converted into an arc. Such mismatching can be caused by variations in chamber pressure, RF power, etc. In this case, a high-temperature torch that occurs at the exit of a gas passage causes thermo-erosion on the surface of the showerhead and funnels the gas passages by creating a nozzle effect in the vicinity of the border between the exit of the passage and the bulk plasma. A consequence of this effect is aerodynamic expansion, turbulence of gas ejected into the chamber, deterioration of uniformity in plasma density, and contamination of the process chamber and especially of the periphery of the showerhead by the deposited erosion products.
On the other hand, a corona discharge causes arcing in the gap on the gas input edges of the gas supply channels, i.e., on the side of the showerhead that faces the cooler plate. This arcing leads to destruction of the showerhead and hence to nonuniform distribution of the process gas in the plasma cavity and to contamination of the process gas and the product with particles of the showerhead material.
It should be further noted that the deterioration described above is not uniform and has a different degree in different areas of the showerhead. For example, gas-directing passages located closer to the periphery of the showerhead deteriorate faster and at a greater degree than in the center of the showerhead. This leads to shortening of the channel lengths in the peripheral part of the showerhead, which results in decrease in gas pressure near the peripheral areas of the process chamber. This, in turn, leads to nonuniformity of plasma.
In order to overcome the drawbacks of the known showerhead cooler systems, the inventors herein have developed a deterioration-resistant system of a showerhead with a gas-feeding cooler plate (hereinafter referred to as “system”) for use in a semiconductor processing chamber that provides uniform distribution of plasma density in the working cavity of the semiconductor processing chamber over the surface of a semiconductor wafer having an increased diameter, e.g., up to 450 mm. This newly developed system is the subject of copending U.S. patent application Ser. No. 14/164,182 filed by the same applicants on Jan. 25, 2014.
It has been found that etching of large-diameter wafers, e.g., 450 mm in diameter, in a CCP plasma treatment apparatus requires that for generating plasma of sufficiently high density a power source of very high frequency up to 150 MHz be used. However, such high frequencies adversely affect distribution of plasma density and lead to nonuniformity of etching. In plasma-treatment apparatuses of the aforementioned type, plasma density depends on the radius of the showerhead and is maximal at the center of the plasma bulk, minimal at the periphery, and linearly decreases in the intermediate part toward the periphery.
In the deterioration-resistant showerhead cooler system of U.S. patent application Ser. No. 14/164,182 uniformity of plasma density is achieved by providing the gas holes of the showerhead with a special geometry that makes it possible to adjust gas permeability of the showerhead. In other words, at uniform permeability of the showerhead, plasma density is changed with the radius of the showerhead and, according to distribution of RF power, is maximal at the center of the showerhead, minimal at the periphery, and linearly decreases in the intermediate area.
In order to prevent the adverse effect of the hollow cathode phenomenon, the inventors herein offer to prevent the erosion of the nozzle and passage surfaces by coating these surfaces with a plasma-resistant coating. It has been experimentally found that coating with a thin film of yttrium oxide or silicon carbide is the best selection for accomplishing the above goal.